The present invention pertains to a method and apparatus for temperature compensation for modulated energy sensors. More particularly, the present invention pertains to a method and apparatus where a thermistor circuit is used to compensate readouts in an energy sensor for ambient temperature variations.
Modulated energy sensors are ones that detect electromagnetic radiation (e.g., infrared radiation, UV radiation, visible light, etc.), modulate the energy and generate an output referenced to the amplitude of the modulated signal. Examples of such modulated energy sensors are thermometers for remote temperature measurements as described in U.S. Pat. Nos. 4,233,512 and 5,391,001. In U.S. Pat. No. 5,391,001 there is a radiation detector, a temperature indicating device connected to the detector, and a modulator disposed in a path of a radiation beam from the object for converting the beam into a series of pulses when the modulator is vibrated in and out of the path. The modulator includes a primary piezoelectric element adapted to vibrate when subjected to driving signals at a frequency related to the resonant frequency of the modulator and having a portion of the primary element at the vibrating tip of the element electrically isolated from the driving element itself which is used to generate an electrical signal rather than respond to an electrical signal. The signal generated provides a measure of the physical motion; producing a maximum voltage at resonance. This signal is used to control the frequency of the driving circuit and seeks the frequency where mechanical resonance occurs, the point of maximum deflection in the horizontal plane. The modulator also includes a blocking element connected to the piezoelectric element and disposed to move in and out of the path of the beam.
In general, modulated optical energy measuring devices rely on alternating current (A.C.) signal amplification, rectification and integration to produce a D.C. signal output representing the energy being measured. There is a need for means to correct the effect of ambient temperature, which, if disregarded, can have a deleterious effect on the instrument's accuracy, stability and measurement repeatability. To counter the effects of ambient temperature variations, a correction signal is generated by employing a temperature sensitive element such as a thermistor that is used in a D.C. network or resistors or similar combinations of passive and active components that can suffer offset and drift complications. A common use for this D.C. compensation signal is to allow it to control gain in a circuit amplifier passing the modulated energy signal. There is a need for a method and apparatus which overcomes the aforementioned deficiencies and compensates for ambient temperature variations in a modulated energy sensing device.